Memory circuitry typically uses sense amplifiers to determine the state of a selected memory cell during a read operation. The memory cell is selectively coupled to a sense bitline. The sense amplifiers typically compare a sensed voltage at a node of the sense bitline with a reference voltage in order to determine the state of the selected memory cell. Varying the reference voltage changes the threshold at which the sense amplifier determines whether the memory cell is storing a one or a zero. Thus the accuracy of the read operation is dependent upon the reference voltage supplied to the sense amplifier.
Floating gate electrically erasable programmable read only memory (FLASH EEPROM) is one example of a type of memory that utilizes sense amplifiers to detect the state of memory cells. A FLASH memory cell is erased when the net charge on the floating gate is neutral. An erased FLASH memory cell is referred to as storing a logical "1". A FLASH memory cell is programmed when the net charge on the floating gate is negative. A programmed FLASH memory cell is referred to as storing a logical "0".
FIG. 1 illustrates typical prior art circuitry for sensing the state of a selected FLASH memory cell. A sense bitline provides a path for sinking current through the selected FLASH memory cell. A reference bitline provides a path for sinking current through a reference FLASH memory cell. A programmed FLASH cell sinks a negligible amount of current, so the reference cell is not programmed. The resulting reference voltage on the reference bitline is compared to a sensed voltage on the sense bitline. The state of the sensed FLASH memory cell is thus compared against the state of the reference FLASH memory cell.
The reference voltage for the sense amplifier is generally derived from Vcc which represents the magnitude of the power supply voltage for the memory circuitry. This reference voltage is generally established at the midpoint of the expected operating range of the memory circuitry to ensure speed, accuracy, and symmetry. Any variation in the power supply voltage will affect the reference voltage for the sense amplifier.
As more standards for computer system power supplies evolve, the memory circuitry is expected to perform over multiple ranges of power supplies. Currently 3.3 volt and 5 volt power supply systems are prevalent in the microprocessor industry. The memory circuitry should maintain the same level of performance regardless of whether a 3.3 volt or a 5 volt power supply is used. Ideally the memory circuitry should maintain the same level of performance over a range of power supply levels in order to account for nominal differences in the level of the power supply voltages as well as tolerance factors. Allowing for a 10% tolerance factor this indicates that the memory circuitry should maintain the same level of performance over a power supply range of approximately 3.0 to 5.5 volts.
Due to the nonlinearities of transistors 110 and 120 in FIG. 1, however, there is a nonlinear relationship between power supply levels and the reference voltage. Increases in Vcc tend to result in disproportionate increases in the reference voltage. In other words, doubling Vcc will not similarly double the reference voltage. The sense amplifier performance tends to degrade if the reference voltage is not at the midpoint of the operating range of the sense bitline voltage.
One disadvantage of the prior art biasing circuitry is that if the memory circuitry is optimized to work at a given power supply level such as 3.3 volts, the memory circuitry may not operate accurately or with the same level of performance at another voltage level such as 5 volts.